Source

File: wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php

protected function prepare_item_for_database( $request ) {
		$prepared_attachment = parent::prepare_item_for_database( $request );

		// Attachment caption (post_excerpt internally).
		if ( isset( $request['caption'] ) ) {
			if ( is_string( $request['caption'] ) ) {
				$prepared_attachment->post_excerpt = $request['caption'];
			} elseif ( isset( $request['caption']['raw'] ) ) {
				$prepared_attachment->post_excerpt = $request['caption']['raw'];
			}
		}

		// Attachment description (post_content internally).
		if ( isset( $request['description'] ) ) {
			if ( is_string( $request['description'] ) ) {
				$prepared_attachment->post_content = $request['description'];
			} elseif ( isset( $request['description']['raw'] ) ) {
				$prepared_attachment->post_content = $request['description']['raw'];
			}
		}

		if ( isset( $request['post'] ) ) {
			$prepared_attachment->post_parent = (int) $request['post'];
		}

		return $prepared_attachment;
	}